This will be a continuation of the investigation of antigenic variation and antibody response in rhinovirus infections. In previous studies we have found that a fairly large proportion of the 90 rhinovirus serotypes are related. This suggests that possibly new types are emerging as antigenic variants of types previously known and that this accounts for the large number of serotypes. Two mechanisms for production of antigenic variants are presence of antibody, producing pressure for antigenic change and mixed infections with more than one serotype which could produce progeny with antigens derived from both types. We have looked at some pairs of reciprocally related serotypes and have noted a complete spectrum of antigenic relationship in the pairs, including intertypes. In previous studies, we have also isolated two serotypes of rhinovirus from the same specimen. We plan to study isolates from subjects with pre-infection antibody, to look for effects of antibody on the isolate, to determine the extent of dual infection, which appears to be as high as 2.3% in our preliminary data, and to study the progeny of experimentally mixed infections in cells. In the course of these experiments we hope to improve the methodology for detecting and identifying these agents, since control of the common cold depends on accurate laboratory diagnosis.